leetcode:二叉树的前中后序遍历

二叉树的前序遍历：根-左-右
方法一：递归

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        self.helper(root,res)
        return res
    def helper(self,root,res) :
        if root :
            res.append(root.val)
            if root.left :
                self.helper(root.left,res)
            if root.right :
                self.helper(root.right,res)

方法二：栈的遍历

class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        white , gray = 0 , 1
        stack = [(white,root)]
        while stack :
            color , node = stack.pop()
            if node == None : continue
            if color == white :   #因为append是从后面加的，所以顺序是反过来的
                stack.append((white,node.right))
                stack.append((white,node.left))
                stack.append((gray,node))
            else :
                res.append(node.val)
        return res

二叉树的中序遍历：左-根-右
方法一：递归

class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        self.helper(root,res)
        return res
        
    def helper(self,root,res) :
        if root :
            if root.left :
                self.helper(root.left,res)
            res.append(root.val)
            if root.right :
                self.helper(root.right,res)

方法二：栈的遍历

class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        white , gray = 0 , 1    #未访问过的  访问过的
        res = []
        stack = [(white,root)]
        while stack :
            color , node = stack.pop()
            if node == None : continue
            if color == white :         #将右子节点，本身，左子节点依次入栈
                stack.append((white,node.right))
                stack.append((gray,node))
                stack.append((white,node.left))
            else:
                res.append(node.val)   #将部分字符类型的数字转化为数字类型，遇到字符则停止
        return res

二叉树的后序遍历：左-右-根
方法一：递归

class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        self.helper(root,res)
        return res
    def helper(self,root,res) :
        if root :
            if root.left :
                self.helper(root.left,res)
            if root.right :
                self.helper(root.right,res)
            res.append(root.val)

方法二：栈的遍历

class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        res = []
        white ,gray = 0 , 1
        stack = [(white,root)]
        while stack :
            color , node = stack.pop()
            if node == None : continue
            if color == white :
                stack.append((gray,node))
                stack.append((white,node.right))
                stack.append((white,node.left))
            else :
                res.append(node.val)
        return res